These studies derive from the very basic chemical question, "How can metal ions (atoms) influence chemical reactions?" Since many of the functions performed by metal ions in living systems involve metal complexes with macrocyclic ligands (porphyrins, chlorophyll, vitamin B12, transferrin, Enniatin B, and monactin), we are seeking to learn what chemical principles are either unique to macrocyclic ligand structures or are maximized (or minimized) by such structures; i.e., what is special about complexes of macrocyclic ligands? Iron and cobalt are the central metal ions in many complexes that enter into oxidation or reduction catalysis in living systems and in various commercial applications. The natural macrocyclic complexes of these elements constitute the most important substances of this class. We are concerned with the ways in which the oxidation reduction properties of these elements respond to carefully controlled variations in the structures of the macrocyclic tetradentate ligands to which they are bound. The pertinent properties include standard potentials, reversibility of electrode reactions, availability or stability of extreme oxidation states and ability of the compound to function in the appropriate reaction; e.g., decompose hydrogen peroxide, oxidize a substrate, form a complex with molecular oxygen, form a stable metal-carbon bond.